1. Field of the Invention
This invention pertains to novel methods and vehicles for the increased dinitrogen fixation of leguminous plants. More specifically, the present invention relates to novel leguminous plant lines, recombinant root nodulating bacteria, and the method of use thereof to control the root nodulation and dinitrogen fixation of leguminous plants.
2. Description of the Prior Art
A major emphasis of recent research in the field of N.sub.2 fixation has been the development and application of bradyrhizobial strains with enhanced N.sub.2 fixation. However, establishing an introduced strain of Bradyrhizobium japonicum "B. japonicum" in the nodules of soybean [Glycine max (L.) Merr.] grown in soils populated by indigenous bradyrhizobia has been, and remains, a critical problem.
Most soils of the United States where a soybean crop has been grown have an indigenous population of the homologous root nodule bacterium, B. japonicum. These indigenous bradyrhizobia are usually excellent competitors for nodulation of soybean in comparison with applied inoculant strains. In the major soybean growing areas of the midwest, the most competitive population of B. japonicum is that of serogroup 123. If improvement in the dinitrogen fixing capacity of the soybean-Bradyrhizobium symbiosis through application of superior strains is to be realized, then the difficult problem of competition from indigenous populations (such as serogroup 123) will have to be solved.
Significant efforts have been made to understand and alter the competitiveness of indigenous bradyrhizobia. For example, attempts to alter soybean nodule occupancy ratios of indigenous versus applied bradyrhizobia have been reported. However, such alterations were only achieved by using ultra-high, economically infeasible rates of the applied strain. In a seven year study, E. P. Dunigan et al. [Agron. J. 76: 463-466 (1984)] demonstrated that the inoculant strain USDA 110 eventually formed the majority of nodules after high rates of application in the first 2 years (serogroup 123 was not among the indigenous population). However, the tenacious competitive ability of serogroup 123 appears not to be related to numbers per se and when normal rates of inoculum are applied the indigenous serogroup 123 population can still form up to 95% of the nodules on soybean.